1. Field of the Invention
The present invention relates to a communication system, a communication apparatus and a communication method of the communication system, and in particular to a technique for transferring data in a system in which a plurality of communication apparatuses are daisy chained.
2. Description of the Related Art
There exist video transfer systems in which a plurality of camera terminals and a controller are connected by a network. For example, a system in which a single controller and a plurality of cameras are daisy chained by a digital bus is disclosed in Japanese Patent Laid-Open No. 2003-69985. Also, a system in which a single camera control apparatus and a plurality of monitoring cameras are connected by a common bus on an AC power line that doubles as a communication line is disclosed in Japanese Patent Laid-Open No. 2004-179784.
In both examples, video data is transmitted reliably and the occurrence of dropped frames is suppressed by performing time-division multiplex communication in which the cameras are designated transfer timeslots to be used, and transmitting the video data of the cameras within the transfer timeslots.
Designating a transfer timeslot for each camera naturally requires that individual identification information or address identification codes such as ID codes or address codes be set in advance for the respective cameras using one method or another. However, usability is adversely affected if the user does the setting manually. Although a method of performing the setting automatically using some sort of protocol is also conceivable, this increases the processing load and complicates the device configuration.
Normally, communication for confirming the device configuration or the like on a network is performed prior to transmission of video data. Time-division multiplex communication performed after designating transfer timeslots cannot be implemented in a state where not even the number of nodes connected on the network is known. Accordingly, the communication protocol used at this time is completely different from the communication protocol used for communications using the transfer timeslots, inevitably complicating the configuration of the protocol stack and increasing the processing load.
Incidentally, when video from a plurality of camera terminals is composed and displayed on a single screen, the video frame cycles of the camera terminals desirably are synchronized. A simple video composition process suffices if the video frame cycles are synchronized, and the isochronism of video captured by the camera terminals is also ensured since the image sensing timing coincides.
Japanese Patent Laid-Open No. 2004-179784 refers to a frame image being output for each allocated transfer timeslot. This only means that the video data of the camera terminals arrives at the controller sequentially in video frames, and does not imply that the video frame cycles of the camera terminals are synchronized. In other words, synchronizing the video frame cycles of the camera terminals has not been taken into account at all in the above prior art.